Bless you Birdie. You can always do what I do if I'm not sure (even after 30 odd years I still come across stuff I don't immediately recognize). Leave it to flower and if you're not happy then pull it up or move it to a another part of the garden where you don't notice it so much and grow it on there to see if it's worth keeping. They're only plants at the end of the day and little one's like that should be easy to get rid of at the end of the day. If you get stuck you'll always get plenty of help on this site. You only have to ask.

Thanks hollytree - I sometimes pot them up and half sink them into a bit of spare earth and watch them from there. That's how I saved Chives, thinking they might be grass, and another flower which self seeds everywhere but whose name I can't remember - might be Aqualegia (tall, thin strong stalks? With blueish, purplish or pinkish flowers?)

Saved some weeds like that once - all that TLC and they refused to even try to help by being 'proper' plants!